Abstract: BACKGROUND: A relationship between higher surgeon volume and lower mortality has been described for breast cancer, but selection bias has not been rigorously evaluated. We studied potential bias in the surgeon volume-outcome relationship by comparing the relationship of surgeon volume to breast cancer mortality and to mortality from other causes of death. METHODS: We conducted an observational cohort study from tumor registry and Medicare claims data on 12 216 women, 66 years or older, with stage I or II breast cancer, who were operated on by 1856 surgeons. Breast cancer mortality and other-cause mortality were determined from death certificate sources and surgeon volume from Medicare claims. RESULTS: Treatment by a high-volume surgeon was associated with younger patient age, white race, less comorbidity, and residence in a more affluent zip code. Patients treated by low-, medium-, and high-volume surgeons had small differences in breast cancer mortality (17.4, 15.7, and 13.0 deaths per 1000 person-years, respectively; P = .03) but larger differences in non-breast cancer mortality (46.0, 36.8, and 31.7 deaths per 1000 person-years, respectively; P < .001). After adjustment for multiple patient and disease factors, women treated by high-volume surgeons, compared with those treated by low-volume surgeons, were not less likely to die of breast cancer (relative risk, 0.94; 95% confidence interval, 0.76-1.16) but were significantly less likely to die of other causes (relative risk, 0.86; 95% confidence interval, 0.75-0.98). CONCLUSIONS: The surgeon volume-outcome relationship for these patients with breast cancer was attributable not to mortality from breast cancer but to other causes of death. The lack of specificity of this relationship raises the possibility of selection bias as an explanatory factor.